[Backup cleanup] remove unused (and no-op) method setBackupProvisioned

Bug: 120120742

Test: 1) atest RunBackupFrameworksServicesRoboTests
2) atest $(find \
frameworks/base/services/tests/servicestests/src/com/android/server/backup \
-name '*Test.java')
3) atest CtsBackupTestCases
4) atest CtsBackupHostTestCases
5) atest GtsBackupTestCases
6) atest GtsBackupHostTestCases

Change-Id: I7b712f346f851ca4f2b6b7d62a0a8fdcc7c77a56